Noble gas isotopes that can be nuclear spin-polarized are very attractive imaging agents because of their high detection sensitivity for nuclear magnetic resonance. In particular, spin-polarized 129Xe enables fast, gas-space imaging of the lungs and airways for clinical diagnosis and physiological studies. Unfortunately, the high cost of 129Xe limits the number of high-resolution lung-imaging studies that can be performed. We propose to develop a system to recover and recycle isotopically enriched xenon gas so that high-resolution lung imaging can become common and inexpensive. The innovation is a cryogenic gas separation system that extracts nearly all the xenon from a patient's exhaled breath and produces a highly pure and sterile product. The recovered xenon can then be repolarized and used for additional imaging procedures. The Phase I project demonstrated the feasibility of the cryogenic approach. Phase II will complete the development of this technology through design, construction, and demonstration of a complete xenon recycling device. Tests will demonstrate the amount of xenon that can be recovered from patients after a lung-imaging procedure. Lung- imaging studies using animal subjects will show conclusively that recycled xenon can produce high-resolution images that cannot be distinguished from images made using "fresh" xenon. PROPOSED COMMERCIAL APPLICATIONS: Rare, noble gas isotopes are a potential breakthrough for medical imaging if they could be used economically. Uses for these isotopes include high-resolution, gas-space imaging of the lungs and airways, studies of blood perfusion and brain physiology, and porous media studies based on diffusion of gaseous isotopes. Technology developed in this program will make these advanced applications practical and economical by enabling these rare isotopes to be reused many times.